The present invention concerns the production of ethylene by the catalytic decomposition of ethyl esters of carboxylic acids.
Ethylene is presently produced essentially by steam cracking of light petroleum fractions.
The depletion of petroleum and the increase of its cost are causing industry to seek to diversify the approaches to the manufacture of ethylene in order to satisfy economically the increasing demand for this product, particularly for ethylene of high purity quality.
The direct synthesis by a FISCHER TROPSCH reaction from the CO/H.sub.2 mixture which is the synthesis gas prepared from coal or natural gas is unfortunately non-selective.
The dehydration of ethanol, a vegetable or sugar fermentation product, leads selectively to ethylene only but at a still prohibitive cost.
The catalytic condensation of methanol, particularly carefully studied on synthetic zeolites by MOBIL OIL CORPORATION and which uses raw material easily accessible from the synthesis gas did not permit reconciling selectivity and productivity.
Another approach to ethylene is the thermal decomposition, or pyrolysis, of ethyl esters. This pyrolysis has long been known and it takes place at temperatures which can reach 500.degree. to 600.degree. C.
For example, in J. Am. Chem. Soc. 61, 1939, 3203, R. F. MAKENS and W. G. EVERSOLE describe the pyrolysis of ethylene formate at a temperature of 375.degree. C. or less. The ethylene formed is not only accompanied by decomposition products of formic acid, but also by a significant quantity of hydrocarbons such as methane, ethane, and butylene.
U.S. Pat. No. 4,270,015 describes obtaining ethylene esters by the catalytic reaction of an aliphatic carboxylic acid containing 2 to 4 carbon atoms with synthesis gas; other than that of ethylene and of the acid from which the ester derives, by pyrolysis of the latter in a quartz reactor at a temperature of the order of practically 450.degree. C.
The ethylene produced contains other hydrocarbons, particularly ethane, as it is also reported by JOHN F. KNIFTON in Journal of Catalysis 79, 147-155 (1983).
The concentration of ethane can reach high values, near 5% by pyrolyzing pure ethyl propionate, at 460.degree. C.
Furthermore, the conversion of the ester and the productivity in terms of ethylene are low.
In order to try to remedy the drawbacks of noncatalytic pyrolysis of ethyl esters, it has been proposed to use catalytic decomposition.
U.S. Pat. No. 4,399,305 describes obtaining high purity ethylene from ethyl acetate with the help of a catalyst composed of a perfluorosulfonic resin commercially sold under the trademark NAFION by duPONT de NEMOURS.
Although the gaseous portion of the product collected after pyrolysis is said to be high purity ethylene, the productivity of the process described remains low as a result of a conversion of the ester not surpassing 20% in the mode of embodiment of the invention which is, however, the best one. Furthermore, no mention is made of the selectivity.
Prior art also informs us that the use of catalysts based on alumina or on silica-alumina does not permit satisfying the present requirements of the industry.
For example, ethyl acetate leads to an ethylene strongly polluted by CO.sub.2 when, according to SENDERENS, Bull. Soc. Chem. France, 1908, 4, NR.3, 826, it is decomposed at 300.degree. C. on alumina and when, according to R. D. OBOLENTSEV and Yu. N. USOV, Doklady Akad. Nauk. SSSR, F1,1950,489, it is decomposed at 400.degree. C. on silica-alumina.